This invention generally relates to mercaptopropionic acid esters and, more particularly, to methods which are especially adapted for providing improved yields of mercaptopropionic acid esters.
U.S. Pat. No. 4,052,440 and 4,067,901 disclose methods for forming relatively high yields of mercaptopropionic acid esters and/or dithio dipropionic acid esters and higher polythio dipropionic acid esters by reaction of acrylic acid esters with hydrogen sulfide in the presence of a weakly basic amine catalyst and a polythio dipropionic acid ester in an amount of at least 30% by weight of the total monothiodipropionic acid ester and polythio dipropionic acid ester present. The polythio dipropionic acid ester is reactive, and takes part in the reaction, with a material improvement in the course of the reaction, and particularly in the yield of the mercaptopropionic acid ester, when the amount of polythio propionic acid ester is at least 30% by weight.
The overall reaction for preparing mercapto propionic acid ester can be set forth by the following equation. ##STR1## In this reaction (1) and in reactions (2), (3), (4) and (5), R is a hydrocarbon group, alkyl, aralkyl, cycloalkyl, or alkylcycloalkyl, having from one to eight carbon atoms.
Under the conditions of the processes disclosed in U.S. Pat. Nos. 4,052,440 and 4,067,901, the above reaction is favored, and the mercaptopropionic acid ester is obtained in good yield. However, the reaction system is extremely complex. A number of other reactions take place concurrently, some of which have been verified, and some of which are postulated from the reaction products that are obtained in the court of the reaction. Under controlled conditions other than those specified for the preparation of mercaptopropionic acid ester, the balance of the reactions can be so controlled that the predominant reaction products is not mercaptopropionic acid ester but dithiodipropionic acid ester, and higher polythiodipropionic acid esters, together with by-products, such as monothiodiproprionic acid esters.
In one important concurrent and consequential side reaction, the mercaptopropionic acid ester reacts with acrylic acid ester to form a monothiodipropionic acid ester, in accordance with the following reaction. ##STR2## Although monothiodipropionic acid esters are useful materials, their concurrent production reduces the yield of the desired mercaptopropionic acid ester. Furthermore, if purity of the mercaptopropionic acid ester is important, the presence of an monothiodipropionic acid ester is a complicating factor because of the difficulty of making a clean separation between the two products.
It is apparent from reaction (2) that in the preparation of the mercaptopropionic acid ester, it is desirable to minimize the formation of monothiodipropionic acid esters. This can theoretically be accomplished in a number of ways. For example, the mercaptopropionic acid ester could be removed as soon as possible, preferably as rapidly as it is formed, from the reaction system, so as to avoid reaction of the product with the acrylic acid ester to form the monothiodipropionic acid ester. This approach can be effective within practical limitations. Another approach is to increase the reaction rate of reaction (1) and thereby reduce the availability of alkyl acrylate for, or otherwise disfavor, reaction (2) and other undesirable side reactions.
U.S. Pat. No. 4,307,225 discloses a reaction of olefinically unsaturated alkyl carboxylates such as methyl acrylate with hydrogen sulfide in the presence of a basic catalyst such as ammonium hydroxide, a lower alcohol such as methanol, and sulfur to yield a mixture of alkyl mercapto carboxylate, dialkyl thiodicarboxylate and dialkyl dithiodicarboxylate, which subsequently undergoes a transesterification reaction with a glycol such as polyether glycol to yield a polymer useful as a sealant.
It has been discovered that the use of a small percentage (approximately 2% by weight) of methanol in the processes of U.S. Pat. Nos. 4,052,440 and 4,067,901 enhances the rate of reaction of methyl acrylate with hydrogen sulfide (reaction (I)) and can suppress some undesirable side products. The lower alcohols such as methanol, however, are not readily acceptable for continuous processes because they tend to distill with the desired product requiring constant addition of methanol to the system or means for recycling the methanol. It has further been discovered that the use of a polyether co-catalyst in the process of U.S. Pat. Nos. 4,052,440 and 4,067,901 provides excellent results without the drawbacks of the lower alcohols.
It is accordingly, a general object of the present invention to provide a method of preparing mercaptopropionic acid esters.
Another object of the present invention is to improve upon the method of preparing mercaptopropionic acid esters disclosed in U.S. Pat. Nos. 4,052,440 and 4,067,901.
Another object of the present invention is to provide an improved continuous process for preparing mercaptopropionic acid esters.
These and other objects of the present invention will be apparent from the following detailed description.